Multi-station processing chambers are used to process several substrates, such as semiconductor wafers, within the same chamber. By processing several wafers at once, the throughput of the system is increased. An example of a multi-station processing chamber is the batch chemical vapor deposition ("CVD") chamber, Concept 2 Altus model, manufactured by Novellus Systems, Inc. located in San Jose, Calif.
While the throughput of multi-station processing chambers is typically greater than single wafer processing chambers, the throughput of a multi-station processing chamber is generally limited by the inability to simultaneously perform incompatible processes. For example, in a CVD tungsten chamber, incompatible reactive gases, such as silane (SiH.sub.4) and tungsten hexafluoride (WF.sub.6) are used within the same chamber. If these reactive gases are permitted to mix at critical SiH.sub.4 to WF.sub.6 ratios, particles may be formed in the gas phase, which then may contaminate the wafers being processed. In conventional CVD tungsten chambers, each wafer introduced to the chamber must be brought up to temperature and exposed to silane gas in an initiation step. If the silane gas is exposed to any tungsten hexafluoride during the initiation step, unwanted particles will form. Thus, the tungsten deposition at other stations can occur only after the newly introduced wafer has completed its heating and silane initiation step.
In addition, where reactive gases, e.g., tungsten hexafluoride, migrate from one area in the chamber to other areas in an uncontrolled fashion prior to the completion of other processing steps, such as the silane initiation, the reactive gases may attack the Ti seed/barrier layers causing damage to the devices on the wafers. This mechanism can be independent of the gas phase particle producing phenomena. Thus, during the heating and silane initiation of the wafer, the other stations in the multi-station chamber must remain idle to avoid the uncontrolled migration of reactive gases or the undesirable combination of reactive gases. Consequently, the throughput of a multi-station processing chamber is limited by the idle period of the different stations while incompatible processes are performed in neighboring stations. Alternatively, a multi-station processing chamber must perform certain incompatible processing steps, such as the silane initiation, in a separate reactor, which adds cost and complexity to the system, and may reduce throughput.